FIG. 1 depicts a conventional direct conversion or homodyne radio-frequency (RF) receiver 100, which may also be heterodyne and preferably low-IF heterodyne.
The antenna 10 converts the radio-frequency electromagnetic (EM) waves into an RF signal, which is first filtered by an RF band-pass filter (BPF) 12. The filtered signal is then amplified by a low-noise amplifier (LNA) 14 in order to increase the strength of the RF signal and reduce the noise Figure of the RF receiver 100. The LNA-amplified signal is next input into a frequency converter represented by the dashed line for being down-converted to baseband, using mixers 16A, 16B and orthogonal signals, i.e. in-phase (I) and quadrature (Q) signals, generated by a so-called local oscillator (LO) 18 and a 90-degree phase shifter (not represented). Each mixer 16A, 16B multiplies the LNA-amplified signal at its RF input with a periodic signal provided at its LO input by the LO 18, which is tuned to the carrier frequency of the desired RF signal. Each frequency-down-converted signal, also called intermediate-frequency (IF) signal, obtained at each IF output of mixers 16A, 16B is respectively filtered by a low-pass IF filter 20A, 20B before being amplified by a respective gain-controlled IF amplifier 22A, 22B. Usually, an IF filter 20A, 20B and its respective IF amplifier 22A, 22B are combined into a single building block as represented by the dashed lines. Each IF-amplified analog signal is then converted into a digital signal by a respective analog-to-digital converter (ADC) 24A, 24B, and the digital signal is afterwards demodulated by the digital baseband (BB) processor 26.
Several types of mixers can be used. However, when the mixers 16A, 16B are unbalanced or single balanced rather than double balanced, the CMOS frequency divider generating the I/Q LO signals produces a lot of 1/f-noise at the mixers outputs, and this is particularly harmful in case of a zero-IF or near-zero-IF receiver. The problem originates from the fact that the LO-signal generator comprises MOS transistors, which components are known to be 1/f-noisy. This causes relatively-slow random fluctuations of the duty cycle and pulse position of the I/Q LO signals generated by the CMOS frequency divider and then amplified by LO buffers. Indeed, a small fraction of the differential I/Q LO signals ends up at the RF input of the mixers 16A, 16B, due to crosstalk via parasitic capacitances around the mixer transistors. Ideally, the fundamental content of these signals exactly cancels out. However, in case of fluctuations and/or static differences in duty cycle and pulse position, a small residual part will be left at the RF input of the mixers 16A, 16B, and will be mixed down to the IF (self mixing). A static mutual deviation in duty cycle and pulse position would result in a DC component at the IF output of the mixers 16A, 16B. However, the deviations are not static but change over time due to the 1/f-noise, such that the IF signal is polluted by 1/f-noise.